Multiple layer printed circuit board

ABSTRACT

A multiple layer printed circuit board and a method of manufacturing multiple layer printed circuit boards which incorporate integral edge shielding in combination with top and bottom shielding to effectively provide a sandwich arrangement within a Faraday Cage. Electromagnetic emissions radiating from an outside surface of either the top or bottom shielding layer are substantially reduced. In one structure, a multiple layer printed circuit board having a sandwich arrangement which includes at least one inner conductive layer for providing a ground plane disposed between the outer shielding layers, the inner conductive layer being electrically connected directly to the edge shield and hence to the outer conductive layers. The larger and more continuous surface area provided by the direct connection to the edge shielding effectively provides an electrical connection having a low inductance and hence in operation results in all ground planes having a more constant non-varying potential.

This invention relates to multiple layer printed circuit boards.

Printed circuit boards have evolved from having signal layers as topand/or bottom layers to structures having a plurality of signal layerssandwiched with insulating layers interleaved between the signal layers.Printed circuit board manufacturers often provide a plurality of metallayers internally disposed within the structure and insulated fromsignal layers for connection to power or ground. The signal and metallayers do not extend out to the edge of the finished printed circuitboard but fall short by a predetermined distance. Thus a small threedimensional unobstructed space around the perimeter of the finishedprinted circuit board is made available for mechanical fasteners tofasten faceplates and printed circuit board stiffeners to the printedcircuit board.

In operation, when the metal layers are connected to power or groundthey are referred to as power or ground planes respectively. Power andground planes provide a convenient way for designers using thistechnology to connect, where required power and/or ground to electroniccomponents mounted on an outermost surface of the multiple layer printedcircuit board and which have leads which are soldered withinPlated-through holes extending through the structure. Signal layerswhich are disposed next to but insulated from a ground plane layer canalso take advantage of well known microstrip or stripline techniqueswhich permit engineers to control the impedance characteristics ofcritical signal tracks located on the signal layer. Multiple layerprinted circuit boards having in the order of ten layers are now commonplace within the telecommunication industry.

Along with this advancement in printed circuit board technology, logicfamilies have been developed which are orders of magnitude faster inoperation than their predecessors. Integrated circuits which operate atthese higher speeds, take less time to switch from a logic zero to alogic one (rise time) and from a logic one to a logic zero (fall time).It is well known within the electronic industry that there is a directcorrelation between the time taken by integrated circuits to switch fromone logic state to the other and the magnitude of electromagneticemissions radiated from the associated signal tracking layer. Simplystated, faster switching times inherently mean greater radiatedelectromagnetic emissions from the signal layers of printed circuitboards having integrated circuits operating at these high speeds.Electromagnetic emissions radiating from a printed circuit board maycorrupt signals in a second neighbouring printed circuit board or theemissions may affect local reception by neighbouring equipment of radioor television signals.

Regulatory bodies in countries around the world have in recent yearsplaced very strict limits on the levels of electromagnetic emissionsallowed to radiate from electronic equipment intended for use withinthese countries. Electronic equipment which does not comply with theseregulated limits generally is not allowed to be used or sold withintheir respective countries.

Manufacturers of electronic equipment have gone to great lengths tocontain and/or limit the levels of electromagnetic emissions radiatingfrom their equipment. Some manufacturers have fabricated metal caseswhich support and effectively encase a complete printed circuit board ina Faraday Cage. Other manufacturers have effectively placed completeshelves of electronic equipment or even frames of equipment in FaradayCages in an attempt to contain radiated electromagnetic emissions fromtheir electronic equipment. Although these methods do attenuate theemissions, they are expensive, they increase the weight and physicalsize of the equipment and still do not solve the problem of sensitiveneighbouring electronic circuitry from being corrupted by radiatedemissions from another electronic circuit within the same frame orshelf.

A more recent approach to attenuating the level of emissions fromelectronic circuitry has been to limit it at its source; the printedcircuit board. Some manufacturers of multiple layer printed circuitboards have attempted to limit electromagnetic emissions by fabricatingtheir printed circuit boards with metal shielding layers which form thetop and bottom outermost layers, the shielding layers in use beingconnected to ground potential. Each of the metal shielding layers has aninside conductive surface and an outside conductive surface.Electromagnetic fields associated with electrical signals originating onsignal layers, may induce radio frequency currents onto the insidesurfaces of the top and/or bottom metal shielding layers. Some of theseradio frequency currents travel solely along the inside surfaces of themetal shielding layers but some eventually find a path to the outsidesurfaces. That portion of a radio frequency current that eventuallytravels along the outside surface of either the top or bottom shieldinglayers will contribute to electromagnetic emissions radiating outwardlyfrom the multiple layer printed circuit board. Many manufacturers usingprinted circuit boards which incorporate top and bottom shielding layersstill opt to enclose shelves or equipment cabinets containing theseprinted circuit boards in effectively a Faraday Cage.

A secondary, yet well known problem associated with prior art multiplelayer printed circuit boards is that of ground plane impedance. Priorart multiple layer printed circuit boards having one or more internalground planes, typically electrically connect the ground planes to themetal shielding layer on each of the top and bottom surfaces using manyplated-through holes disposed in predetermined patterns across thesurfaces of the printed circuit boards. Plated-through holes exhibit ahigh inductance connection and when in operation, these ground planeswhen connected in this manner very often are not all at the same voltagepotential. It is desirable to have all ground planes at one commonnon-varying potential considering that these planes serve to provide areference for sensitive electronic devices. Another concern of usingmultiple plated-through holes to provide the electrical connectionbetween internal ground planes and the metal shielding layers is thatthese holes extend through the printed circuit board, and hence resultin obstructions when engineers route signal tracks on signal layerswithin the printed circuit board.

The present invention seeks to provide a multiple layer printed circuitboard which minimizes the above problems.

According to one aspect of the invention there is provided a multiplelayer printed circuit board comprising a sandwich arrangement of twoouter conductive layers, an electrically conducting signal layer andinsulating layers disposed between the conducting layers and the signallayer, the outer conductive layers disposed so as to provide top andbottom shielding layers of the sandwich arrangement; and integral edgeshielding means comprising a conductive edge shielding layer providedupon at least one edge of the sandwich arrangement, extending to theouter conductive layers and electrically connected directly to each ofthe outer conductive layers.

The invention defined above extends to all forms of multiple-layerprinted circuit boards including back planes or panels used inelectronic systems.

Where all signal layers of a multiple layer printed circuit board areused to track signals operating at high frequencies then the sandwicharrangement may be comprised by the whole printed circuit board.However, it is possible for the multiple layer printed circuit board tohave a certain signal layer or layers dedicated to tracking just highfrequency signals and another signal layer or layers dedicated totracking all other required signals which would not significantlycontribute to electromagnetic emissions. In such a case, the sandwicharrangement would be provided by certain contiguous layers, i.e. thosesignal layers dedicated to tracking the high frequency signals andassociated insulating and outer conductive layers of the sandwicharrangement. The sandwich arrangement would then include only some ofthe total layers of the printed circuit board and each outer conductivelayer or shielding layer of the sandwich arrangement could be disposedas an interior layer of the whole printed circuit board. All otherlayers could be disposed outside the sandwich arrangement and segregatedfrom it by an insulating layer.

Advantageously the sandwich arrangement includes at least one innerconductive layer for providing a ground plane disposed between the outershielding layers, the inner conductive layer being electricallyconnected directly to the edge shielding means and hence to the outerconductive layers. In operation, ground planes connected in this mannerprovide a ground plane having a low impedance connection to ground (i.e.to the outer shielding layers which are grounded in operation). Thelarger and more continuous surface area provided by the directconnection to the edge shielding means effectively provides anelectrical connection having a low inductance and hence in operationresults in all ground planes having a more constant non-varyingpotential. A second advantage resulting from connecting inner groundplanes directly to the edge shielding means, is that the directelectrical connection does not provide any obstructions to engineerswhen routing signal tracks on signal layers within the printed circuitboard during a design phase.

The edge shielding means may be provided solely on one edge or onseveral edges of the sandwich arrangement. The degree of shielding toemissions from induced radio frequency currents does of course dependupon the amount of edge shielding provided.

Preferably, however, the multiple layer printed circuit board comprisesedge shielding means on all edge surfaces of the sandwich arrangementthus containing the majority of the induced radio frequency currents totravelling solely on the inside surfaces of the top and bottom shieldinglayers, provided by the two outer conductive layers and hence reducingemissions.

Providing edge shielding on all edge surfaces of a sandwich arrangementof a multiple layer printed circuit board in combination with the topand bottom shielding layers substantially and effectively provides thesandwich arrangement with an integral Faraday Cage. In use of a printedcircuit board having edge shielding on all edge surfaces,electromagnetic emissions radiating from an outside surface of eitherthe top or bottom shielding layer are substantially reduced bycontaining induced radio frequency currents to travelling along only aninside surface of the top and bottom shielding layers. Induced highfrequency currents travelling along the inside surface of the top andbottom shielding layers are substantially contained within the sandwicharrangement of the multiple layer printed circuit board by the shieldingprovided by the combination of top and bottom shielding layers and theedge shielding means. Hence, potential emissions resulting from inducedradio frequency currents reaching the outside surfaces of the top andbottom shielding layers are minimized.

In one practical arrangement of a multiple layer printed circuit boardthe shielding material on the edge surface covers the entire edgesurface except for a plurality of spaced apart edge surface regionswhich are devoid of the shielding means, each edge surface region havinga sufficiently small width dimension to provide a desired attenuationbarrier to high frequency energy. A width dimension of approximately 0.3centimeters provides about thirty decibels of attenuation to highfrequency energy in the order of one gigahertz.

Conveniently the practical arrangement may include aperture meansextending through the sandwich arrangement, and conductor meansextending through the aperture means and electrically interconnectingtop and bottom conductive layers, the aperture means being disposedinwardly of at least one edge surface region devoid of shielding meansand located to provide an attenuation barrier to high frequency energydirected toward the surface region. The aperture means may be comprisedby a plurality of holes extending through the sandwich arrangement in apredetermined pattern, and the conductor means comprises an electricalconductor provided upon the inner surface of each hole. Providingaperture means inward of the edge regions devoid of conducting materialfurther reduces the amount of radio frequency current which reaches theoutside surfaces of the top and/or bottom metal shielding layers of theprinted circuit board.

In another aspect of the invention there is provided a method ofmanufacturing a printed circuit board having multiple layers in asandwich arrangement, the method comprising the steps of: providing aprocess panel having a central region representing the position of theprinted circuit board and a border region outside of the central region,the process panel comprising a sandwich of signal layers and insulatinglayers between the signal layers; forming circuit board boundaryapertures through the process panel, the apertures disposed in spacedapart positions around the central region to define bridges between theborder region and the central region and with a part of each surface ofeach aperture providing part of an edge for the layers of the printedcircuit board; providing a conductive material on all surfaces of theprocess panel including surfaces of the boundary apertures and bridges;and severing the printed circuit board from the process panel by cuttingacross the bridges from boundary aperture to boundary aperture to freethe printed circuit board from the process panel, with the conductivematerial forming outer conductive layers as shielding layers of thesandwich arrangement of the printed circuit board and the conductivematerial on said surface parts of the boundary apertures forming edgeshielding means on the parts of the aperture surfaces which form partsof the edge of the printed circuit board layers with edge regionsproduced by cutting the bridges being devoid of the conductive material.

In yet a further aspect of the invention there is provided a method ofmanufacturing a printed circuit board having multiple layers in asandwich arrangement, the method comprising the steps of: providing aprocess panel having a central region representing the position of theprinted circuit board and a border region outside of the central region,the process panel comprising a sandwich of signal layers and insulatinglayers between the signal layers; forming circuit board boundaryapertures through the process panel, the apertures disposed in spacedapart positions around the central region to define bridges between theborder region and the central region and with a part of each surface ofeach aperture providing part of an edge for the layers of the printedcircuit board; providing an electroless coating of copper on allsurfaces of the process panel including surfaces of the boundaryapertures and the bridges; forming an electroplated layer of copper uponall the surfaces of the electroless coated copper, by passing anelectric current into the electroless coating on the border region, thecurrent passing from the border region to the central region by passingalong the electroless coating of the bridges, and along the electrolesscoating of the central region, the current also travelling along theelectroless coating of the boundary apertures; and severing the circuitboard from the process panel by cutting across the bridges from boundaryaperture to boundary aperture, the layer of copper forming outerconductive shielding layers as part of the sandwich arrangement of theprinted circuit board and the layer of copper also forming edgeshielding means on the parts of the aperture surfaces which form partsof the edge of the circuit board layers with edge regions produced bycutting the bridges, being devoid of copper.

Manufacturing printed circuit boards and incorporating an electroplatingprocess to provide the shielding on both the major surfaces and theedges is both economical and effective. The addition of one extra stepto an otherwise conventional printed circuit board manufacturing processenables manufacturers of multiple layer printed circuit boards toprovide an effective and substantially continuous shield around theprinted circuit boards. Any added drilling such as for the aperturemeans need not be a separate step but an addition to an already existingdrilling step.

In performance of the method, it is advantageous to provide the processpanel with a conductive layer interleaved with the signal and insulatinglayers, the conductive layer extending outwardly into the border regionin at least one location. Formation of the circuit board boundaryapertures then exposes an edge of the conductive layer. The electrolesscoating of copper is then formed upon this exposed edge to provide anelectrical connection with the subsequently applied electroplated layer.Hence, an electrical connection is provided between the conductive layerand each of the outer layers whereby plated-through holes which normallyserve for grounding purposes, may be avoided thereby allowing forgreater freedom of printed circuit board design.

One embodiment of the invention will now be described, by way ofexample, with reference to the accompanying drawings in which:

FIG. 1 is an isometric cross-sectional view of part of a end of a priorart multiple layer printed circuit board;

FIG. 2 is a plan view of the prior art multiple layer printed circuitboard during manufacture;

FIG. 3 is an end view of a multiple layer printed circuit board of theembodiment;

FIG. 4 is an isometric cross-sectional view of the multiple layerprinted circuit board of FIG. 3 and taken along the line 4--4 of FIG. 3;

FIG. 5 is a plan view of the multiple layer printed circuit board of theembodiment at one stage during to manufacture; and

FIG. 6 is a cross-sectional view of the multiple layer printed circuitboard of FIG. 5 at the manufacturing stage and taken along the line 6--6of FIG. 5.

As may be seen in FIG. 1, a prior art multiple layer printed circuitboard 10 comprises a sandwich arrangement having multiple internalsignal layers 12, conductive layers 14 for use as power planes, andconductive layers 15 for use as ground planes. The signal layers 12 andthe conductive layers 14, 15 are separated one from the other byinsulating layers 16. All signal layers 12 and conductive layers 14, 15do not extend out to the edges 19 of the finished printed circuit board10 but fall short by a predetermined distance. This smallthree-dimensional unobstructed space forming the perimeter region of thefinished printed circuit board is made available for mechanicalfasteners to fasten faceplates and printed circuit board stiffeners to.To provide some attenuation to electromagnetic emissions radiating frominternal signal layers 12, top and bottom outermost layers 20, 22 of thesandwich arrangement are also conductive layers. The top and bottomoutermost conductive layers 20, 22 are typically connected to electricalground potential when the circuit board is in operation. Plated-throughholes 24 extending through the printed circuit board 10 are used toelectrically connect the top outermost conductive layer 20 to the bottomoutermost conductive layer 22 and to connect in known manner, thoseinternal conductive layers 15 which are to be used as ground planes,when in operation, to the top and bottom outermost conductive layers 20,22. Clearance holes in the layers 12 and 14 are provided also in knownmanner around the holes 24 to electrically isolate these layers from theplated-through holes. Integrated circuit positions 18 (shown asrectangular outlines in outermost layer 20) have associated holes 28which are plated with a conductive layer such as copper, to provideelectrical connection between integrated circuit terminal pins (notshown) and either signal layers 12 or conductive layers 14, 15 asrequired.

During manufacture of a multiple layer printed circuit board 10 it isconventional to use a process panel 30 (FIG. 2) from which the printedcircuit board is produced. This process panel 30 is larger in plan viewthan the final multiple layer printed circuit board 10 (shown in chaindotted in FIG. 2), the printed circuit board 10 being surrounded in theprocess panel 30 by a continuous and integral border region 32. Theborder region 32 is required for handling the printed circuit board 10during manufacture as will now be described. The border region 32 mayconveniently be used for registration purposes during the drilling ofall holes on the printed circuit board 10 and for this purpose may haveregistration holes 35. Accurate component hole location is criticalconsidering that the majority of modern printed circuit boards haveassociated circuit components inserted into position with the use ofrobotics.

A conventional electroless process step is required for the purpose ofpreparing the process panel 30 for a subsequent and conventionalelectroplating step which deposits a layer of copper onto all outersurfaces of the process panel 30 including outer perimeter edges of theborder region 32 and as well on all surfaces of holes 28, 24. To enablethe electroless process step to be performed the process panel 30 isheld in a vertical position in a bath containing a copper suspension bya clamping arrangement 33 (FIG. 2) attached to the border region 32 ofthe process panel 30. During the subsequent electroplating step, anelectrode clamp (mechanically similar to the clamping arrangement 33) isaffixed to the border region 32 along one edge of the process panel 30to support the process panel 30 while suspended in a copper solutionbath and to pass electric current to the surface of the process panel.

After completion of all manufacturing steps the multiple layer printedcircuit board 10 is severed (along the chain dotted line in FIG. 2) fromthe process panel 30 and the border region 32 is discarded. Theresulting multiple layer printed circuit board 10 (FIG. 1) has the topand bottom outermost layers 20, 22 which provide copper surfaces of thesandwich arrangement but where severed, around the chain dotted line ofFIG. 2, exposed edges 19, exist which are devoid of copper.

Apart from the fact that no edge shielding is provided on thisconventional board and emissions resulting from induced radio frequencycurrents freely take place, the whole board manufacturing process is onewhich is dependent upon no edge shielding being provided.

In an embodiment now to be described, parts of the structure similar tothe prior art structure of FIGS. 1 and 2, carry the same referencenumerals for convenience.

In the embodiment of the invention as shown in FIGS. 3 and 4, a multiplelayer printed circuit board 60 comprises a sandwich arrangement havingmultiple internal signal layers 12, conductive layers 70 for use asground planes and conductive layers 14 for use as power planes, all ofwhich are separated one from the other by insulating layers 16. Top andbottom outermost layers 20, 22 of the sandwich arrangement are alsoconductive layers and are for connection to electrical ground potentialwhen in operation. Integrated circuit positions 18 have associatedplated-through holes 28 which provide electrical connection betweenintegrated circuit terminal pins (not shown) and either signal layers12, or conductive layers 14,70 as required. The multiple layer printedcircuit board 60 differs from the prior art, in a manner now to bedescribed.

The multiple layer printed circuit board 60 differs basically from theprior art in that it is provided with integral edge shielding means.This is provided upon each edge of the printed circuit board by aconductive edge shielding layer 43 which extends along each edge so asto cover each entire edge surface 50 of the sandwich arrangement exceptfor a plurality of small spaced-apart edge regions 52 which are devoidof conducting material. The conductive edge shielding layer 43 of theedge surfaces 50 extends to the top and bottom outermost conductivelayers 20, 22 and is electrically connected directly to each of theouter conductive layers. Except for the presence of edge regions 52, acontinuous electrical connection is formed at the junction of theconductive edge shielding layer 43 and the conductive top and bottomoutermost layers 20, 22. Aperture means in the form of plated-throughshielding holes 46 extend through the sandwich arrangement, electricallyconnecting the top and bottom outermost conductive layers 20, 22 and arepositioned in a predetermined pattern immediately inward of the edgeregions 52 as is shown in FIG. 3. The shielding holes 46 are positionedin two rows, with the holes of the rows being staggered from row to rowalong the lengths of the rows.

The edge regions 52 are devoid of conducting material for reasonsrelating to a process or method of manufacturing multiple layer printedcircuit boards, yet to be described. The edge regions 52 being devoid ofconductive material effectively form a break in the conductive shieldinglayer 43 but if the width of these edge regions 52 is kept to a minimum,effective shielding is still achieved. Edge regions having a width ofabout 0.3 centimeters provide more than 30 decibels of attenuation tohigh frequency energy in the order of about one gigahertz (1×10*9 Hz).The potential for further attenuation is possible by the addition of theshielding holes 46. The spacing between the shielding holes can easilybe made to be significantly smaller than the width of the edge regions52 as they are simply drilled holes and hence if positioned in a manneras shown in FIGS. 3 or 5 and inward of the edge regions 52, theyeffectively reduce the break or opening in the conductive shieldinglayer 43 formed by the edge regions 52 and thus further reduce thechance of induced radio frequency currents travelling on the insidesurface of either the top or bottom conductive outermost surfaces fromreaching and travelling along the outside surfaces and contributing toradiated emissions.

Internally, the signal layers 12 and conductive layers 14 do not extendto the edges of the board 60 and are thus similar to the prior artstructure of FIGS. 1 and 2. Thus they are spaced apart from the edgeshielding layer 43 as shown by FIG. 4. However, in the embodiment, theconductive layers 70 intended to be used as ground planes do extendoutwardly to the edge surfaces 50 to be electrically connected with andmerge into the edge shielding layer 43.

The multiple layer printed circuit board 60 is made from a process panel72, as shown in FIG. 5 and is basically constructed in layers usingconventional techniques. One significant difference in construction ofthe process panel 72 is that all conductive layers 70 to be used asground planes are provided by original process panel layers 70a which,as shown in chain dot in FIG. 5 and in solid in FIG. 6, extend beyondthe final dimensions of the multiple layer printed circuit board 60(shown in chain dot in FIGS. 5 and 6) and into a border region 74. Allconductive layers 14 and signal layers 12 stop short in conventionalfashion of the final dimensions of the multiple layer printed circuitboard 60.

The process panel in a substantially finished stage would include allholes 28, 46 such as for component terminal pins, shielding,connectivity purposes and mechanical fasteners. etc., to be drilled inthe appropriate positions.

Predetermined locations on the process panel 72 are routed in the borderregion 74 to provide circuit board boundary apertures. These are in theform of a plurality of in-series slots 40 which extend through thesandwich arrangement. The slots 40 are located and orientated such thattheir inwardly disposed side surfaces 42 form parts of edges 50 of thefinal multiple layer printed circuit board 60. Formation of the slots 40cuts through and exposes cut edges of all conductive layers 70a alongthe inwardly disposed side surface 42 of each slot 40. The side surfaces42 are longer than the widths of the bridges 44. Predetermined spaces orbridges 44 between adjacent slots 40 are required so that the printedcircuit board 60 may still be structurally supported by the processpanel 72, during the remainder of the manufacturing process. Aperturemeans, in the form of shielding holes 46 are drilled through thesandwich arrangement in a predetermined pattern immediately inward ofthe bridges 44 as is shown in FIGS. 5 and 6. The shielding holes 46 arepositioned in two rows, with the holes of the rows being staggered aspreviously mentioned.

An electroless step is performed which provides a conductive coating onall external surfaces of the process panel 72 including the surfaces ofall slots 40 and the surfaces of all apertures including the shieldingholes 46. The electroless process step prepares the process panel 72 fora subsequent electroplating step.

The electroplating step involves securing an electrode clampingarrangement (not shown) similar to that shown in FIG. 2, onto the borderregion 74, submersing the process panel 72 into a bath containing forexample, a copper solution and passing a current to the surface of theprocess panel which is sufficient for electroplating. Electroplating theprocess panel 72 provides all exposed surfaces, (i.e. the conductivecoating) of the panel 72 to be plated with a substantial layer ofcopper. For example the electroplating step provides the top and bottomoutermost conductive layers 20, 22; it provides a conductive layer onthe surfaces of the slots 40 which is integral with the layers 20, 22 asthey are formed simultaneously. In addition, the copper layers on theinwardly disposed surfaces 42 of the slots, forms an electrical contactwith the exposed cut edges of all conductive layers 70 along theinwardly disposed surface 42 of each slot 40. The electroplating stepwould also provide the conductive means through the aperture means byeffectively plating the surfaces of the shielding holes 46.

Freeing the multiple layer printed circuit board 60 from the processpanel 72 is achieved by simply severing across each bridge 44 on each ofthe four sides of the multiple layer printed circuit board 60 along aline which is colinear with the inward disposed surface 42 of acorresponding slot 40. The finished multiple layer printed circuit board60 of FIG. 3 has edge surfaces 50 coated with conductive edge shieldinglayers 43 formed by the plating on inward disposed surfaces 42 of theslots 40 and severed edge regions 52 produced across the bridges 44 andwhich are devoid of conductive material and hence expose a small portionof the sandwich arrangement; the conductive (electroplated) edgeshielding layers 43 being integral with the top and bottom conductivelayers 20, 22.

The number of bridges 44, or more importantly the total surface area ofthe bridges 44 should be sufficient to provide a low resistance path forrequired electroplating current to pass from the electrode clamps (notshown) via the border region 74, to form the top and bottom outermostlayers 20, 22 and to electroplate the surfaces 42 of the slots 40. Aspreviously stated, it is desirable that for reduced emissions, the widthof the bridges 44 or the spacing between the slots be kept to a minimumso as to produce narrow edge regions 52. From an emissions point of viewit is preferable to have multiple slots having relatively smallassociated bridges than it is to have a few longer slots and havinglarger bridges.

The outermost layers 20, 22 of the sandwich arrangement need not be theoutermost surfaces of the multiple-layer printed circuit board. In afurther embodiment of the invention (not shown) a multiple-layer printedcircuit board has one or more insulating or signal layers disposedoutside the sandwich arrangement. It is conceivable to only requireshielding of certain signal layers which in operation would be likely tocontribute to emissions. Such signal layers would be part of thesandwich arrangement and thus shielded while signal layers which inoperation would be less likely to contribute to emissions and/orinsulating layers could lie outside the sandwich arrangement to form therest of the thickness of the printed circuit board.

As an alternative to the aperture means or shielding holes 46 or incombination with the shielding holes 46, the regions devoid ofconductive material 52 could easily be coated in a separate process stepwith a copper (conductive) paint-like material to effectively formcontinuous edge shielding which is also integral with the top and bottomsurfaces 20, 22.

In a further embodiment (not shown), in a multiple layer printed circuitboard, the multiple shielding holes 46 are replaced with a narrow slotdisposed inward of the region of the edge region 52 and which overlapportions of the edge shielding layer 43 on either side of the regions52.

In operation the combined shielding provided by the top and bottomoutermost layers 20, 22 and the edge shielding provided by the edgeshielding layer 43 and by the shielding holes 46 substantially containinduced high frequency currents within the sandwich and hence reduceradiated emissions from the multiple layer printed circuit board.

In operation, conductive layers 70 for use as ground planes which areconnected directly to the edge shielding layer 43 and hence areconnected to the outer shielding layers 20, 22 provide ground planeshaving low impedance paths to ground potential. The large andsubstantially continuous surface area provided by the direct connectionto the edge shielding layers of the layers 70 results in all groundplanes within a multiple layer printed circuit board, when in operation,having a more constant and non-varying potential from one ground planeto the next. As well, directly connecting ground planes in this mannerdoes not provide any obstructions to a designer during a design phase,when routing signal tracks on signal layers of the multiple layerprinted circuit board. In other words, plated-through holes for groundconnection of ground planes, are avoided.

We claim:
 1. A multiple layer printed circuit board comprising:asandwich arrangement of two outer conductive layers, an electricallyconducting signal layer and insulating layers disposed between theconductive layers and the signal layer, the outer conductive layersdisposed so as to provide top and bottom shielding layers of thesandwich arrangement; and integral edge shielding means comprising aconductive edge shielding layer provided upon at least one edge of thesandwich arrangement, extending to the outer conductive layers andelectrically connected directly to each of the outer conductive layers.2. A printed circuit board as claimed in claim 1 wherein the sandwicharrangement further comprises at least one inner conductive layer forproviding a ground plane, the inner conductive layer being physicallyspaced apart from the outer conductive layers and from the signal layerby insulating layers, and being electrically and physically connecteddirectly to the edge shielding means.
 3. A printed circuit board asclaimed in claim 1 wherein the edge shielding means is provided on alledge surfaces of the sandwich arrangement.
 4. A printed circuit board asclaimed in claim 2 wherein the edge shielding means is provided on alledge surfaces of the sandwich arrangement.
 5. A multiple layer printedcircuit board comprising:a sandwich arrangement of two outer conductivelayers, an electrically conducting signal layer and insulating layersdisposed between the conductive layers and the signal layer, the outerconductive layers disposed so as to provide top and bottom shieldinglayers of the sandwich arrangement; and integral edge shielding meanscomprising a conductive edge shielding layer provided upon all edgesurfaces of the sandwich arrangement, extending to the outer conductivelayers and electrically connected directly to each of the outerconductive layers and wherein the edge shielding means covers eachentire edge surface except for a plurality of spaced apart edge surfaceregions which are devoid of the shielding means, each edge surfaceregion having a sufficiently small width dimension to provide a desiredattenuation barrier to high frequency energy.
 6. A printed circuit boardas claimed in claim 5 wherein the width dimension around the boardperiphery of each edge surface region which is devoid of shielding meansis less than about 0.3 cm to provide at least about 30 decibels ofattenuation to high frequency energy in the order of one gigahertz.
 7. Aprinted circuit board as claimed in claim 5 and having aperture meansextending through the sandwich arrangement, and conductor meansextending through the aperture means and electrically interconnectingtop and bottom conductive layers, the aperture means being disposedinwardly of at least one edge surface region devoid of shielding meansand located to provide an attenuation barrier to high frequency energydirected toward said surface region.
 8. A printed circuit board asclaimed in claim 7 wherein with respect to said at least one edgesurface region, the aperture means comprises a plurality of holesextending through the sandwich arrangement in a predetermined pattern,and the conductor means comprises an electrical conductor provided uponthe inner surface of each hole.
 9. A multiple layer printed circuitboard comprising:a sandwich arrangement of two outer conductive layers,an electrically conducting signal layer and insulating layers disposedbetween the conductive layers and the signal layer, the outer conductivelayers disposed so as to provide top and bottom shielding layers of thesandwich arrangement, the sandwich arrangement further comprising atleast one inner conductive layer for providing a ground plane, the innerconductive layer being physically spaced apart from the outer conductivelayers and from the signal layer by insulating layers; and integral edgeshielding means comprising a conductive edge shielding layer providedupon all edge surfaces of the sandwich arrangement, extending to theouter conductive layers and electrically connected directly to each ofthe outer conductive layers and to the inner conductive layer whichextends directly to the edge shielding means and wherein the edgeshielding means covers each entire edge surface except for a pluralityof spaced apart edge surface regions which are devoid of the shieldingmeans, each edge surface region having a sufficiently small widthdimension to provide a desired attenuation barrier to high frequencyenergy.
 10. A printed circuit board as claimed in claim 9 wherein thewidth dimension around the board periphery of each edge surface regionwhich is devoid of shielding means is less than about 0.3 cm to provideat least about 30 decibels of attenuation to high frequency energy inthe order of one gigahertz.
 11. A printed circuit board as claimed inclaim 9 and having aperture means extending through the sandwicharrangement, and conductor means extending through the aperture meansand electrically interconnecting top and bottom conductive layers, theaperture means being disposed inwardly of at least one edge surfaceregion devoid of shielding means and located to provide an attenuationbarrier to high frequency energy directed toward said surface region.12. A printed circuit board as claimed in claim 11 wherein with respectto said at least one edge surface region, the aperture means comprises aplurality of holes extending through the sandwich arrangement in apredetermined pattern, and the conductor means comprises an electricalconductor provided upon the inner surface of each hole.